PostgreSQL 8.4.7 Documentation
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# F.13. hstore

This module implements a data type `hstore` for storing sets of (key,value) pairs within a single PostgreSQL data field. This can be useful in various scenarios, such as rows with many attributes that are rarely examined, or semi-structured data.

In the current implementation, neither the key nor the value string can exceed 65535 bytes in length; an error will be thrown if this limit is exceeded. These maximum lengths may change in future releases.

## F.13.1. `hstore` External Representation

The text representation of an `hstore` value includes zero or more `key` `=>` `value` items, separated by commas. For example:

        k => v
        foo => bar, baz => whatever
        "1-a" => "anything at all"

The order of the items is not considered significant (and may not be reproduced on output). Whitespace between items or around the `=>` sign is ignored. Use double quotes if a key or value includes whitespace, comma, `=` or `>`. To include a double quote or a backslash in a key or value, precede it with another backslash. (Keep in mind that depending on the setting of `standard_conforming_strings`, you may need to double backslashes in SQL literal strings.)

A value (but not a key) can be a SQL NULL. This is represented as

        key => NULL

The `NULL` keyword is not case-sensitive. Again, use double quotes if you want the string `null` to be treated as an ordinary data value.

Currently, double quotes are always used to surround key and value strings on output, even when this is not strictly necessary.

## F.13.2. `hstore` Operators and Functions

Table F-5. `hstore` Operators

Operator | Description | Example | Result
--- | --- | --- | ---
`hstore` `->` `text` | get value for key (null if not present) | `'a=>x, b=>y'::hstore -> 'a'` | `x`
`text` `=>` `text` | make single-item `hstore` | `'a' => 'b'` | `"a"=>"b"`
`hstore` `||` `hstore` | concatenation | `'a=>b, c=>d'::hstore || 'c=>x, d=>q'::hstore` | `"a"=>"b", "c"=>"x", "d"=>"q"`
`hstore` `?` `text` | does `hstore` contain key? | `'a=>1'::hstore ? 'a'` | `t`
`hstore` `@>` `hstore` | does left operand contain right? | `'a=>b, b=>1, c=>NULL'::hstore @> 'b=>1'` | `t`
`hstore` `<@` `hstore` | is left operand contained in right? | `'a=>c'::hstore <@ 'a=>b, b=>1, c=>NULL'` | `f`

(Before PostgreSQL 8.2, the containment operators @> and <@ were respectively called @ and ~. These names are still available, but are deprecated and will eventually be retired. Notice that the old names are reversed from the convention formerly followed by the core geometric datatypes!)

> Note: The `=>` operator is deprecated and may be removed in a future release. Use the `hstore(text, text)` function instead.

Table F-6. `hstore` Functions

Function | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
`hstore(text, text)` | `hstore` | make single-item `hstore` | `hstore('a', 'b')` | `"a"=>"b"`
`akeys(hstore)` | `text[]` | get `hstore`'s keys as array | `akeys('a=>1,b=>2')` | `{a,b}`

`skeys(hstore)`

`setof text`

get `hstore`'s keys as set

`skeys('a=>1,b=>2')`

    a
    b

`avals(hstore)` | `text[]` | get `hstore`'s values as array | `avals('a=>1,b=>2')` | `{1,2}`

`svals(hstore)`

`setof text`

get `hstore`'s values as set

`svals('a=>1,b=>2')`

    1
    2

`each(hstore)`

`setof (key text, value text)`

get `hstore`'s keys and values as set

`select * from each('a=>1,b=>2')`

     key | value
    -----+-------
     a   | 1
     b   | 2

`exist(hstore,text)` | `boolean` | does `hstore` contain key? | `exist('a=>1','a')` | `t`
`defined(hstore,text)` | `boolean` | does `hstore` contain non-null value for key? | `defined('a=>NULL','a')` | `f`
`delete(hstore,text)` | `hstore` | delete any item matching key | `delete('a=>1,b=>2','b')` | `"a"=>"1"`

## F.13.3. Indexes

`hstore` has index support for `@>` and `?` operators. You can use either GiST or GIN index types. For example:

    CREATE INDEX hidx ON testhstore USING GIST(h);

    CREATE INDEX hidx ON testhstore USING GIN(h);

## F.13.4. Examples

Add a key, or update an existing key with a new value:

    UPDATE tab SET h = h || ('c' => '3');

Delete a key:

    UPDATE tab SET h = delete(h, 'k1');

## F.13.5. Statistics

The `hstore` type, because of its intrinsic liberality, could contain a lot of different keys. Checking for valid keys is the task of the application. Examples below demonstrate several techniques for checking keys and obtaining statistics.

Simple example:

    SELECT * FROM each('aaa=>bq, b=>NULL, ""=>1');

Using a table:

    SELECT (each(h)).key, (each(h)).value INTO stat FROM testhstore;

Online statistics:

    SELECT key, count(*) FROM
      (SELECT (each(h)).key FROM testhstore) AS stat
      GROUP BY key
      ORDER BY count DESC, key;
        key    | count
    -----------+-------
     line      |   883
     query     |   207
     pos       |   203
     node      |   202
     space     |   197
     status    |   195
     public    |   194
     title     |   190
     org       |   189
    ...................

## F.13.6. Authors

Oleg Bartunov `<oleg@sai.msu.su>`, Moscow, Moscow University, Russia

Teodor Sigaev `<teodor@sigaev.ru>`, Moscow, Delta-Soft Ltd., Russia

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